System and method for communicating data using symbol-based randomized orthogonal frequency division multiplexing (OFDM) with applied frequency domain spreading

ABSTRACT

A device and system communicates data and includes a modulation and mapping circuit that modulates and maps data symbols into a plurality of multiple subcarrier frequencies that are orthogonal to each other to form an Orthogonal Frequency Division Multiplexed (OFDM) communications signal based on a fixed or variable OFDM symbol rate. A pseudo-random signal generator operative with the modulation and mapping circuit generates pseudo-random signals to the modulation and mapping circuit based on an encryption algorithm for frequency hopping each subcarrier at an OFDM symbol rate to lower any probability of interception and detection, reduce power per frequency (dB/Hz/sec), and lower any required transmission power while maintaining an instantaneous signal-to-noise ratio A frequency domain spreader circuit is operatively connected to the modulation and mapping circuit for spreading the multiple subcarriers over the frequency domain.

FIELD OF THE INVENTION

The present invention relates to communications systems, and moreparticularly, this invention relates to multiple carrier communicationsystems, including but not limited to, Orthogonal Frequency DivisionMultiplexing (OFDM) communications systems.

BACKGROUND OF THE INVENTION

In OFDM communications systems the frequencies and modulation of afrequency-division multiplexing (FDM) communications signal are arrangedorthogonal with each other to eliminate interference between signals oneach frequency. In this system, low-rate modulations with relativelylong symbols compared to the channel time characteristics are lesssensitive to multipath propagation issues OFDM thus transmits a numberof low symbol-rate data streams on separate narrow frequency subbandsusing multiple frequencies simultaneously instead of transmitting asingle, high symbol-rate stream on one wide frequency band on a singlefrequency. These multiple subbands have the advantage that the channelpropagation effects are generally more constant over a given subbandthan over the entire channel as a whole. A classical In-phase/Quadrature(I/Q) modulation can be transmitted over individual subbands. Also, OFDMis typically used in conjunction with a Forward Error Correction scheme,which in this instance, is sometimes termed Coded Orthogonal FDM orCOFDM.

An OFDM signal can be considered the sum of a number of orthogonalsubcarrier signals, with baseband data on each individual subcarrierindependently modulated, for example, by Quadrature Amplitude Modulation(QAM) or Phase-Shift Keying (PSK). This baseband signal can alsomodulate a main RF carrier.

OFDM communications systems have high spectrum efficiency (a high numberof bits per second per Hz of bandwidth), simple mitigation of multi-pathinterference, and an ease in filtering noise. OFDM communicationssystems suffer, however, from time-variations in the channel, especiallythose which cause carrier frequency offsets. Because the OFDM signal isthe sum of a large number of subcarrier signals, it can have a highpeak-to-average amplitude or power ratio. It is also necessary tominimize intermodulation between subcarrier signals, which can createself-interference in-band, and create adjacent channel interference.Carrier phase noise, Doppler frequency shifts, and clock jitter cancreate Inter-Carrier Interference (ICI) for closely frequency-spacedsubcarriers. The subcarriers are typically transmitted at assignedfrequency locations within a transmission spectrum. Over the duration ofthe transmission of an OFDM signal, the average power per subcarrier issignificant, and can be easily detected and intercepted, which isundesirable to a system requiring Low Probability of Detection (LPD) andLow Probability of Interception (LPI) characteristics. The receiver thatis to receive the OFDM signal requires a minimum signal-to-noise ratio(SNR) per subcarrier in order to demodulate and decode the signal withan acceptably low bit error rate (BER). If there is other unwantedenergy within the transmission spectrum, the SNR can decrease causing anincrease in BER. Said unwanted energy can be unintentional noise fromother sources. In this case the noise is referred to as “interference”and the sources are referred to as “interferers.” If the unwanted energycorrupting the transmission is transmitted intentionally by some thirdparty source known as a jammer, it is referred to as a jamming signal.The conventional OFDM signal is susceptible to such interferers andjammers because of the required minimum SNR per subcarrier for anacceptably low BER. Further, frequency selective fading in the channelcauses transmission nulls within the OFDM signal's transmissionspectrum, which selectively reduce the SNR on certain subcarriers withinthose nulls, depending on their frequency location, leading to anundesirable increase in HER.

SUMMARY OF THE INVENTION

A device and system communicates data and includes a modulation andmapping circuit that modulates and maps data symbols into a plurality ofmultiple subcarrier frequencies that are orthogonal to each other toform an Orthogonal Frequency Division Multiplexed (OFDM) communicationssignal based on a fixed or variable OFDM symbol rate. A pseudo-randomsignal generator operative with the modulation and mapping circuitgenerates pseudo-random signals to the modulation and mapping circuitbased on an encryption algorithm for frequency hopping each subcarrierat an OFDM symbol rate to lower any probability of interception anddetection, reduce power per frequency (dB/Hz/sec), and lower anyrequired transmission power while maintaining an instantaneoussignal-to-noise ratio, A frequency domain spreader circuit isoperatively connected to the modulation and mapping circuit forspreading the multiple subcarriers over the frequency domain.

The frequency domain spreader can be formed as a Walsh Transform circuitthat is operative for applying a Walsh Transform and spreading themultiple subcarriers over the frequency domain. This Walsh Transformcircuit is operative for multiplying an input vector of a symbol by theWalsh Transform. An Inverse Fast Fourier Transform (IFFT) circuit ispositioned to receive signals from the frequency domain spreadercircuit. The pseudo-random signal generator is operative for generatinga pseudo-random signal based on the encryption algorithm such thatconsecutive OFDM symbols do not transmit subcarriers on the samefrequency. The pseudo-random signal generator is operative forgenerating a pseudo-random signal based on the encryption algorithm suchthat OFDM symbols do not transmit subcarriers on adjacent frequenciesfor reduced Inter-Carrier Interference (ICI). The pseudo-random signalgenerator can also be operative for generating a pseudo-random signalbased on the encryption algorithm such that a guard interval is reducedor eliminated.

In yet another aspect, the modulation and mapping circuit is operativefor inserting signals for reducing peak-to-average power ratio (PAPR). Amodulator can map the communications data into modulated symbols basedon a specific mapping algorithm. The pseudo-random signal generator isoperatively connected to the modulator for varying amplitude and phasevalues using an encryption algorithm. An encoder as a forward errorcorrection circuit can add a FEC code.

In yet another aspect, the device can be part of a transmitter thattransmits the communications signal that carries communications data. Areceiver can receive the communications signal and include a demappingand demodulation circuit and frequency domain despreading circuit forprocessing the communications signal to obtain the communications data.The transmitter is operative for modulating a main carrier signal.

In yet another aspect, a method is disclosed in which communicationsdata is distributed over multiple subcarriers that are orthogonal toeach other to form an Orthogonal Frequency Division Multiplexed (OFDM)communications signal based on a fixed or variable OFDM symbol rate.Each subcarrier can be frequency hopped at an OFDM symbol rate to lowerany probability of interception and detection, reduce power perfrequency (dB/Hz/sec), and lower any required transmission power whilemaintaining an instantaneous signal-to-noise ratio. The multiplesubcarriers are spread over the frequency domain and the communicationsdata is transmitted over the signal,

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features, and advantages of the present invention willbecome apparent from the detailed description of the invention whichfollows, when considered in light of the accompanying drawings in which:

FIGS. 1A and 1B are prior art, high-level block diagrams showing arespective transmitter and receiver circuits for an IEEE 802.11a OFDMmodem connected through a radio transmission channel.

FIGS. 2A through 2C are spectrum graphs representing a) a single carriersignal; b) a Frequency Division Multiplexing (FDM) signal; and c) anOrthogonal Frequency Division Multiplexing (OFDM) signal.

FIG. 3A is a graph showing a three-dimensional representation of aconventional OFDM signal.

FIG. 3B is a spectrogram showing a power distribution for an example ofa conventional OFDM signal such as shown in FIG. 3A.

FIG. 3C is a graph showing a two-dimensional representation of a typicalOFDM spectrum such as shown in FIG. 3A.

FIG. 3D is graph for a 64-QAM constellation of a conventional OFDMsignal such as is shown in FIG. 3A.

FIGS. 4 a-4 d are spectral density graphs with each graph showing anOFDM spectrum, with one graph showing 52 subcarriers ON, and comparingthis graph with a graph of spectrum in which a respective 26 subcarriersare ON, 13 subcarriers are ON, and 6 subcarriers are ON producing areduced Inter-Carrier Interference (ICI). FIG. 4 e is an explanation andequation representing total transmit power from a spectral densityfunction.

FIG. 5 is a graph showing a three-dimensional spectrogram (power vs.frequency vs. time) of Symbol-Based and Frequency Randomized subcarriersfor the frequency hopping OFDM signal in accordance with a non-limitingexample of the present invention, and also showing a conventional singlecarrier signal overlaid for comparison.

FIG. 6 is a high-level block diagram of a transmitter that can be usedfor generating the frequency hopping OFDM signal in accordance with anon-limiting example of the present invention.

FIG. 7 is a high-level block diagram of a receiver that can be used forreceiving and processing the transmitted frequency hopping OFDM signalin accordance with a non-limiting example of the present invention.

FIGS. 8A and 8B are graphs showing a spectral comparison for thefrequency hopping OFDM signal and showing the LPD improvement inaccordance with a non-limiting example of the present invention.

FIG. 9 are graphs showing a spectral comparison in noise for thefrequency hopping OFDM signal and a signal modified to reduce itsdetectability in accordance with a non-limiting example of the presentinvention.

FIG. 10 is a graph showing a three-dimensional representation of thefrequency hopping OFDM signal before a Walsh transform in accordancewith a non-limiting example of the present invention.

FIG. 11 is a graph showing a three-dimensional representation of thefrequency hopping OFDM signal after the Walsh transform in accordancewith a non-limiting example of the present invention.

FIG. 12 is a graph of power vs. frequency showing the addition of theWalsh transform to the subcarriers of the frequency hopping OFDM signalin which subcarriers are symbol-based and randomized in accordance witha non-limiting example of the present invention.

FIG. 13 is a graph showing a three-dimensional representation of thereceived frequency hopping OFDM signal before the Inverse Walshtransform.

FIG. 14 is a graph showing a three-dimensional representation of thereceived frequency hopping OFDM signal after the Inverse Walsh transformand also showing the received signal constellation.

FIGS. 15A and 15B are graphs showing a three-dimensional representationof the frequency hopping OFDM signal before and after the WalshTransform and for illustration and comparison purposes showing a singlecarrier in the middle of the band.

FIG. 16 is a graph showing a three-dimensional representation of areceived frequency hopping OFDM signal with an interfering signal afterfrequency-domain despreading in accordance with a non-limiting exampleof the present invention.

FIG. 17 is a graph showing a three-dimensional representation of thefrequency hopping OFDM signal before the Walsh transform in which noiseis added.

FIG. 18 is a graph showing a three-dimensional representation of areceived frequency hopping OFDM signal with an interferer before theInverse Walsh transform in accordance with a non-limiting example of thepresent invention.

FIG. 19 is a graph showing a spectral comparison of the frequencyhopping OFDM signal with an interferer, illustrating the spectrum withgraphical representations when the Walsh transform is ON and OFF inaccordance with a non-limiting example of the present invention.

FIG. 20 is a graph showing the power spectrum of the received frequencyhopping OFDM signal with an interferer before the Inverse Walshtransform.

FIG. 21 is a graph showing the frequency-domain despreading of thefrequency hopping OFDM signal with an interferer in accordance with anon-limiting example of the present invention.

FIG. 22 is a graph showing a three-dimensional representation of thefrequency hopping OFDM signal with frequency-domain despreading and withan interferer and also showing a received signal constellation inaccordance with a non-limiting example of the present invention.

FIG. 23 is a block diagram of an example of a communications system thatcan be used in accordance with a non-limiting example of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

The system, apparatus and method in accordance with a non-limitingexample of the present invention uses a Symbol Based Randomization(SBR), Orthogonal Frequency Division Multiplexing (OFDM) communicationssignal to enhance the Low Probability of Interception (LPI) and LowProbability of Detection (LPD). This signal also allows an increasedtransmit power within a Federal Communications Commission (FCC) spectralmask by reducing the average power per Hertz per second whilemaintaining the same instantaneous signal-to-noise ratio (SNR). Afrequency-domain spreading function, such as the Walsh transform, canalso be applied in the frequency domain to enhance performance.

Orthogonal Frequency Division Multiplexing (OFDM) is also termedMulticarrier Modulation (MCM) because the signal uses multiple carriersignals that are transmitted at different frequencies. Some of the bitsor symbols normally transmitted on one channel or carrier are nowtransmitted by this system on multiple carriers in the channel. AdvancedDigital Signal Processing (DSP) techniques distribute the data overmultiple carriers (subcarriers) at predetermined frequencies. Forexample, if the lowest-frequency subcarrier uses a base frequency, theother subcarriers could be integer multiples of that base frequency. Theparticular relationship among the subcarriers is considered theorthogonality such that the energy from one subcarrier can appear at afrequency where all other subcarrier's energy equal zero. There can be asuperposition of frequencies in the same frequency range. This resultsin a lower symbol rate on each subcarrier with less Inter-SymbolInterference (ISI) due to adverse effects of multipath. In many OFDMcommunications systems, a Guard Interval (GI) or Cyclic Prefix (CP) isprefixed or appended to the OFDM symbol to mitigate the effects of ISI.

FIGS. 1A and 1B are high-level block diagrams showing basic circuitcomponents of an IEEE 802.11a OFDM modem, and showing the transmittercircuit 30 in FIG. 1A and the receiver circuit 32 in FIG. 1B. Thetransmitter circuit 30 (also termed “transmitter” for clarity) transmitsan OFDM signal as shown in FIG. 2C. By comparison, FIG. 2A shows thespectrum of a single carrier signal and FIG. 2B shows in comparison tothe single carrier signal of FIG. 2A, the spectrum of a classicalFrequency Division Multiplexing (FDM) signal. FIG. 2C shows the spectrumof an OFDM signal.

The drawings in FIG. 2A through 2C show that OFDM is based on afrequency-division multiplexing (FDM) system where each frequencychannel is modulated. The frequencies and modulation of an FDM systemare now orthogonal to each other to eliminate interference betweenchannels. Because low-rate modulations with relatively long symbolscompared to the channel time characteristics are less sensitive tomultipath, an OFDM communications system allows a number of low-ratesymbol streams to be transmitted simultaneously on multiple carriersrather than having one high-rate symbol stream transmitted on a singlecarrier. Thus, the frequency spectrum in an OFDM communications systemis divided into multiple low-bandwidth subbands. Since each subbandcovers a relatively narrow section of the frequency spectrum, channelpropagation effects are more constant or “flat” over a given subbandcompared to channel variations over the entire occupied spectrum. Anytype of in-phase and quadrature (I/Q) modulation can be used to modulateany subcarrier, for example, Binary Phase Shift Keying (BPSK),Quadrature Phase Shift Keying (QPSK) or Quadrature Amplitude Modulation(QAM), or any of the numerous and different derivations of thesemodulation schemes. Different signal processing techniques, for example,channel coding, power allocation, adaptive modulation encoding, andsimilar schemes can be applied to one or more subbands. Multi-userallocation is also possible for example using time, coding, or frequencyseparation.

In an OFDM communications system using a transmitter and receiver suchas shown in FIGS. 1A and 1B, one transmitter will transmit a signal ondozens or thousands of different orthogonal frequencies that areindependent with respect to the relative amplitude and phaserelationship between the frequencies. Each subcarrier signal typicallywill have space for only a single narrowband signal because the signalsare closely spaced and it is important to prevent signals on adjacentsubcarriers from interfering with each other. In an OFDM system, thesymbols on each subcarrier are constructed such that energy from theirfrequency components are zero at the center of every other subcarrier,enabling a higher spectral efficiency for OFDM symbols than is possiblein classic FDM.

The OFDM system as shown in FIGS. 1A and 1B includes channel coding as aForward Error Correction (FEC) technique, using a Forward ErrorCorrection encoder to create a coded orthogonal FDM (COFDM) signal.Channel-State Information (CSI) techniques can also be employed,including continuous wave (CW) interferer and/or selective channelsystems.

An OFDM signal is typically the sum of each of the orthogonalsubcarriers. Baseband data is independently modulated onto each of theorthogonal subcarriers using some type of modulation, such as QuadratureAmplitude Modulation (QAM) or Phase Shift Keying (PSK) schemes asdiscussed before. Because the spectrum of each subcarrier overlaps, itcan be considerably wider than if no overlap were allowed. Thus, OFDMprovides high spectrum efficiency. Because each subcarrier operates at alow symbol rate, the duration of each symbol in the subcarrier is long.(For clarity, “symbol rate” is equal to the inverse of “symbolduration”). By using Forward Error Correction (FEC) equalization andmodulation, there can be an enhanced resistance against a) linkdispersion, b) slowly changing phase distortion and fading, c) frequencyresponse nulls, d) constant interference, and e) burst noise. Further,the use of a Guard Interval (GI) or cyclic prefix provides enhancedresistance against multipath in the transmission channel.

Typically, in OFDM communications system, a subcarrier and somewhatrectangular pulse can be employed and operative by an Inverse DiscreteFourier Transform (IDFT) using an Inverse Fast Fourier Transform (IFFT)circuit within the transmitter. At a receiver, a Fast Fourier Transform(FFT) circuit reverses this operation. The rectangular pulse shaperesults in a Sin(x)/x spectrum in the subcarriers.

The spacing of subcarriers can be chosen such that the receivedsubcarriers can cause zero or acceptably low Inter-Carrier Interference(ICI) when the receiver and transmitter are synchronized. Typically,OFDM communications systems split the available bandwidth into manynarrow-band subbands from as little as a few dozen to as many as eightthousand to ten thousand. Unlike the communications system providingmultiple channels using classical FDM as in FIG. 2 b, the subcarriersfor each subband in OFDM are orthogonal to each other and have closespacing and little overhead. In an OFDM communications system, there isalso little overhead associated with any switching that may occurbetween users as in a Time Division Multiplexing Access (TDMA)communications system. Usually, the orthogonality of subcarriers in anOFDM communications system allows each carrier to have an integer numberof cycles over a symbol period. As a result, the spectrum of asubcarrier has a null at the center frequency of its adjacentsubcarriers.

Usually, in an OFDM communications system, the spectrum required fortransmitting data is chosen based on the input data and a desiredmodulation scheme to be used with each carrier that is assigned the datato transmit. Any amplitude and phase of the carrier is calculated basedon the modulation, for example, BPSK, QPSK or QAM as noted before. Anyrequired spectrum is converted using the IFFT circuit to ensure carriersignals are orthogonal.

It should be understood that a FFT circuit transforms a cyclic timedomain signal to an equivalent frequency spectrum by finding anequivalent waveform that is generated as a sum of orthogonal sinusoidalcomponents. The frequency spectrum of the time domain signal is usuallyrepresented by the amplitude and phase sinusoidal components. The IFFTcircuit performs the reverse process and transforms the spectrum of theamplitude and phase into a time domain signal. For example, an IFFTcircuit can convert a set of complex data points into a time domainsignal of the same number of points. Each complex input point willresult in an integral number of sinusoid and cosinusoid cyclesrepresented by the same number of points as were input to the IFFT. Eachsinusoid known as the in-phase component, and cosinusoid known as thequadrature component, will be orthogonal to all other componentsgenerated by the IFFT. Thus, orthogonal carriers can be generated bysetting an amplitude and phase for each frequency point representing adesired subcarrier frequency and performing the IFFT.

It should be understood that a Guard Interval (GI), also termed a cyclicprefix, often is added to an OFDM symbol. The guard interval reduces theeffects of the wireless channel on Inter-Symbol Interference (ISI) andcontains redundant transmission information. Referring to the IEEE802.11a standard as a non-limiting example, if a carrier spacing is 3125KHz, and the Fourier Transforms are performed over 3.2 microseconds,then a 0.8 microsecond guard interval can be applied for ISI rejection.The guard “interval” could be the last T_(g) seconds of an active symbolperiod that is prefixed to an OFDM symbol, making it a cyclic prefix. Itis kept short for a fraction of “T,” corresponding to the total lengthof the active symbol, yet longer than the channel impulse response. Thishelps reduce the ISI and Inter-Carrier Interference (ICI) and maintainssubcarrier orthogonality. In this example, a time waveform appearsperiodic to the receiver over the duration of the FFT.

To reduce ICI, the OFDM symbol can be cyclically extended in the guardtime to ensure that delayed replicas of the OFDM symbol can have aninteger number of cycles within the FFT interval, as long as the delayis smaller than the guard time. As a result, multipath signals withdelays smaller than the guard time would not produce ICI.

Multipath interference is caused when multiple copies of the transmittedsignal arrive at the receiver at different times. It should beunderstood that an OFDM communications system reduces the effect ofmultipath interference by providing the ability to add signal redundancyin both frequency and time by the use of various coding algorithms. Forexample, with the IEEE 802.11a standard using OFDM, 48 carriers can betransmitted simultaneously. The coding gain can be provided using aone-half (½.) convolutional encoder at the transmitter and later aViterbi decoder. Data bits can be interleaved across multiple symbolsand carriers. Lost data often is recoverable because of interleavingacross the frequency and time space.

Increasing the data rate requires an increase in the symbol rate for afixed number of carriers, fixed modulation scheme and fixed sample rate.For a single carrier system, complex equalizers and adaptive filters arerequired at the receiver to compensate for the magnitude and timedistortions caused by the channel. The accuracy and dynamic rangerequired of such equalizers and filters increases markedly as symboltimes are decreased. However, in an OFDM system, for example, when 48subcarriers are transmitted simultaneously, the symbol rate iseffectively reduced by 48 times, significantly reducing the requirementsof channel equalizers and filters. The reduced symbol rate of an OFDMsystem enables a robust communication link, resistant to ISI.

It should be understood that an OFDM receiver receives a sum of thedifferent signals as subcarriers. The addition of a guard interval canfurther enhance performance in an OFDM system by ensuring that no symboltransitions occur during each received symbol time. For example, if anOFDM subcarrier is BPSK modulated, there would be a 180 degree phasejump at symbol boundaries. By choosing a guard interval that is longerthan the largest expected time difference between the first and lastmultipath signals, such phase transitions can occur only during theguard time, meaning there are no phase transitions during the FFTinterval. If the phase transitions of a delayed path occur within theFFT interval of the receiver, then the summation of the subcarriers ofthe first path with the phase modulated waves of the delayed path wouldno longer produce a set of orthogonal subcarriers, resulting in acertain level of interference.

FIG. 1A illustrates a high-level block diagram of the prior arttransmitter 30 for the IEEE 802.11a OFDM modem described above, andincludes a Forward Error Correction (FEC) Coder circuit 34 that receivesa signal representing the data to be communicated 33, and encodes thesignal with a forward error correction code as described above. Thesignal passes to an interleaving and mapping circuit 36 in whichinterleaving and frequency mapping occurs. An IFFT circuit 38 receivesthe interleaved and frequency mapped signal and creates multiple timedomain carriers summed in a single in-phase/quadrature time domainsequence known as a symbol. A guard interval circuit 40 adds the guardinterval. A symbol wave shaping circuit 42, for example a raised cosinefilter, shapes the symbol waveform to limit its spectral content.Afterward, an In-phase/Quadrature (I/Q) modulator 44 processes thebaseband I/Q signal, producing I/Q modulation, and also receiving aLocal Oscillator (LO) signal from LO signal generator 46. Signalup-conversion to the final transmit carrier frequency occurs at mixer48, which receives a local oscillator signal generated by LO signalgenerator 50. Afterward, the signal is amplified by a High PowerAmplifier (HPA) 52, and the OFDM signal is transmitted through anantenna 54 on its carrier wave into the RF channel 31. Various stages offrequency filtering, for example between the I/Q Modulator 44 and mixer48, and between the mixer 48 and HPA 52, and at the output of the HPA 52are not shown in the block diagram.

FIG. 1B shows a high-level block diagram of the prior art receivercircuit 32 used in the exemplary IEEE 802.11a OFDM modem. The antenna 60receives the OFDM signal from the RF Channel 31 on the carrier wave. Itis amplified within a low noise amplifier (LNA) 62. Signaldown-conversion occurs within a mixer 64, which also receives a localoscillator signal generated by an LO signal generator 66. An AutomaticGain Control (AGC) amplifier 68 provides automatic gain control to thedown-converted signal to ensure the appropriate signal level is appliedto the subsequent circuitry. The AGC circuit uses a feedback techniqueand is well known to those skilled in the art. In-phase and quadraturesignal detection occurs within an I/Q Detect circuit 70, which alsoreceives a local oscillator signal generated from a LO signal generator72, which is also operative with an Automatic Frequency Control (AFC)clock recovery circuit 74, as illustrated. The AFC circuit adjusts thelocal oscillator 72 frequency to keep the I/Q detector tunedappropriately. The I/Q Detect circuit 70, AFC clock Recovery circuit 74,and LO signal generator 72 form a feedback loop as illustrated and knownto those skilled in the art. The guard interval is removed within a GIcircuit 76. The Fast Fourier Transform (FFT) is applied on thesubcarriers as a reverse of the IFFT within an FFT circuit 78. Demappingand deinterleaving occur within a Demapping and Deinterleaving circuit80. Forward error correction decoding occurs within an FEC decoder 82,which finishes the signal processing and recovers the original data asreceived communications data 83. It is thus evident that the function ofthe receiver circuit 32 as shown in FIG. 1B operates in a mannerfunctionally the reverse of the transmitter circuit 30 shown in FIG. 1A.

As discussed above, OFDM communications systems can use FEC techniquesand known interleaving and mapping techniques before IFFT processing asshown in FIG. 1A, and demapping and deinterleaving techniques followedby FEC decoding after FET processing as shown in FIG. 1B.

These interleaving, coding, e.g., convolutional codes, includingpuncturing, and deinterleaving and decoding and related techniques oftenare integral parts of OFDM communications systems. As an example, a rate½, K=7 convolutional code can be used as an industry standard code forforward error correction (FEC) during encoding. For purposes ofunderstanding the present invention, a more detailed description ofthese basic system components now follows A convolutional code is anerror-correcting code, and usually has three parameters (n, k, m) with nequal to the number of output bits, k equal to the number of input bits,and m equal to the number of memory registers, in one non-limitingexample. The quantity k/n could be called the code rate with thisdefinition and is a measure of the efficiency of the code. K and nparameters range typically from 1 to 8, m ranges typically from 2 to 10,and the code rate typically ranges from ⅛ to ⅞ in non-limiting examples.Sometimes convolutional code chips are specified by parameters (n, k, L)with L equal to the constraint length of the code. Thus, the constraintlength can represent the number of bits in an encoder memory that wouldaffect the generation of n output bits. Sometimes the letters may beswitched depending on the definitions used.

The transformation of the encoded data is a function of the informationsymbols and the constraint length of the code. Single bit input codescan produce punctured codes that give different code rates. For example,when a rate ½ code is used, the transmission of a subset of the outputbits of the encoder can convert the rate ½ code into a rate ⅔ code.Thus, one hardware circuit or module can produce codes of differentrates. Punctured codes can be used also, which allow rates to be changeddynamically through software or hardware depending on channelconditions, such as rain or other channel impairing conditions.

An encoder for a convolutional code typically uses a Linear FeedbackShift Register (LFSR) or look-up table (LUT) for encoding, which usuallyincludes an input bit as well as a number of previous input bits (knownas the state of the encoder), the table value being the output bit orbits of the encoder. It is possible to view the encoder function as astate diagram, a tree diagram or a trellis diagram.

Decoding systems for convolutional codes can use 1) sequential decoding,or 2) maximum likelihood decoding, such as Viterbi decoding in onenon-limiting example, which typically is more desirable. Sequentialdecoding allows both forward and backward movement through the trellis.Viterbi decoding as maximum likelihood decoding examines a receivesequence of given length, computes a metric for each path, and makes adecision based on the metric. Turbo codes are another example of aforward error correction scheme that can be used.

Puncturing convolutional codes is a common practice in some OFDM systemsand can be used in accordance with non-limiting examples of the presentinvention. It should be understood that in some examples a puncturedconvolutional code is a higher rate code obtained by the periodicelimination of specific logic bits or symbols from the output of a lowrate encoder. Punctured convolutional code performance can be degradedcompared with original codes, but typically the data rate increases.

Some of the basic components that could be used as non-limiting examplesof the present invention include the transmitter as described beforethat incorporates a convolutional encoder, which encodes a sequence ofbinary input vectors to produce the sequence of binary output vectorsand can be defined using a trellis structure. An interleaver, forexample, a block interleaver, can permute the bits of the outputvectors. The interleaved data would also be modulated at the transmitter(by mapping to transmit symbols) and transmitted. At a receiver, ademodulator demodulates the signal.

A block deinterleaver recovers the bits that were interleaved. A Viterbidecoder could decode the deinterleaved bit soft decisions to producebinary output data.

Often a Viterbi forward error correction module or core is used thatwould include a convolutional encoder and Viterbi decoder as part of aradio modem or transceiver as described above. For example if theconstraint length of the convolutional code is 7, the encoder andViterbi decoder could support selectable code rates of ½, ⅔, ¾, ⅘, ⅚,6/7, ⅞ using industry standard puncturing algorithms.

Different design and block systems parameters could include theconstraint length as a number of input bits over which the convolutionalcode is computed, and a convolutional code rate as the ratio of theinput to output bits for the convolutional encoder. The puncturing ratecould include a ratio of input to output bits for the convolutionalencoder using the puncturing process, for example, derived from a rate ½code.

The Viterbi decoder parameters could include the convolutional code rateas a ratio of input to output bits for the convolutional encoder. Thepuncture rate could be the ratio of input to output bits for theconvolutional encoder using a puncturing process and can be derived froma rate ½ mother code. The input bits could be the number of processingbits for the decoder. The Viterbi input width could be the width ofinput data (i.e. soft decisions) to the Viterbi decoder A metricregister length could be the width of registers storing the metrics. Atrace back depth could be the length of path required by the Viterbidecoder to compute the most likely decoded bit value. The size of thememory storing the path metrics information for the decoding processcould be the memory size. In some instances, a Viterbi decoder couldinclude a First-In/First-Out (FIFO) buffer between depuncture andViterbi function blocks or modules. The Viterbi output width could bethe width of input data to the Viterbi decoder.

The encoder could include a puncturing block circuit or module as notedabove. Usually a convolutional encoder may have a constraint length of 7and take the form of a shift register with a number of elements, forexample, 6. One bit can be input for each clock cycle. Thus, the outputbits could be defined by a combination of shift register elements usinga standard generator code and be concatenated to form an encoded outputsequence. There could be a serial or parallel byte data interface at theinput. The output width could be programmable depending on the puncturedcode rate of the application.

A Viterbi decoder in non-limiting examples could divide the input datastream into blocks, and estimate the most likely data sequence. Eachdecoded data sequence could be output in a burst. The input andcalculations can be continuous and require four clock cycles for everytwo bits of data in one non-limiting example. An input FIFO can bedependent on a depuncture input data rate.

Also turbo codes could be used as high-performance error correctioncodes or low-density parity-check codes that approach the Shannon limitas the theoretical limit of maximum information transfer rate over anoisy channel. Thus, some available bandwidth can be increased withoutincreasing the power of the transmission. Instead of producing binarydigits from the signal, the front-end of the decoder could be designedto produce a likelihood measure for each bit.

FIGS. 3A through 3D are graphs showing different representations of aconventional OFDM signal, such as produced by the prior art OFDM modemtransmitter 30 shown in FIG. 1A.

FIG. 3A is a graph showing a three-dimensional representation of theOFDM signal with the frequency along one axis, time in seconds alonganother axis, and the “magnitude” or power on the vertical axis, forminga graph that indicates a magnitude vs. frequency vs. timerepresentation. It is evident from FIG. 3A that the OFDM signal can bedetected in the frequency domain. FIG. 3B is a graph showing aspectrogram or power distribution of the OFDM signal shown in FIG. 3A.FIG. 3C is a graph representing a two-dimensional OFDM spectrum of thethree-dimensional OFDM signal shown in FIG. 3A. FIG. 3D shows a 64-QAMconstellation for the OFDM signal shown in FIG. 3A. These graphstogether depict the power distributed over multiple subcarriers. FIG. 4Eis an explanation and equation representing total transmit power form aspectral density function.

FIGS. 4A through 4D are graphs showing a representation of the OFDMsignal spectrum with different frequency subcarriers turned ON and OFF.In the upper left graph (FIG. 4A), the OFDM signal spectrum shows all 52carriers turned ON, indicating in this non-limiting example an IEEE802.11a standard using 52 carriers. In the upper right (FIG. 4B) 26subcarriers are ON, showing the transmit power having a three decibelincrease over the 52 carrier case in FIG. 4 a, due to 26 carriers(subcarriers) being turned OFF. It should be understood that the totaltransmit power is equal to the area under the curve of the powerspectral density function. The lower left and lower right graphs (FIGS.4C and 4D) show thirteen subcarriers and six subcarriers turned ONrespectively. There is transmit power increase of 6 decibels (6 dB) with13 carriers turned ON and a 9 decibel (9 dB) increase in transmit powerwith 6 subcarriers turned ON. Because the peak power is 6 dB higher inthe 13 subcarrier case, the distance in which the signal will be useable(for a free-space channel) will be doubled. Further increases in rangemay be realized as peak power is increased. FIG. 4 d illustrates reducedInter-Carrier Interference (ICI) due to the wide spacing of thesubcarriers.

It should be understood that OFDM coded transmissions may be easilydetected and received by unintended recipients by detection of datasubcarriers and pilot tones. Addition of multiple sine waves or carrierswith random amplitudes and phases to the waveform will cause it toapproach a Gaussian distribution due to the central limit theorem. Asignal having a Gaussian random distribution inherently has an enhancedLow Probability of Interception (LPI) and Low Probability of Detection(LPD) because it appears similar to additive white Gaussian noise (AWGN)to a receiver.

In accordance with a non-limiting example of the present invention, amodified transmitter as explained in detail below uses an IFFT to createmultiple subcarriers located at specific frequencies. Only a smallsubset of the possible carriers need to be used at any one time toenhance power, reduce ICI, and enhance LPI and LPD. Subcarrier centerfrequencies can be changed at OFDM symbol times according to anencryption algorithm. Such an algorithm can generate a pseudo-randomfrequency hopping sequence and frequency hopping subcarriers inaccordance with a non-limiting example of the present invention. Thus,fast-frequency hopping can change the subcarriers frequency for eachOFDM symbol, and provide a one thousand (1,000) times faster frequencyhopping than the Bluetooth standard, and ten times its data rate.Additional benefits can include a reduced ICI, a reduced ISI, andreduced transmitter overhead from the guard interval. The system,apparatus and method in accordance with a non-limiting example of thepresent invention allows a symbol-based randomization for the OFDMsignal.

A Walsh transform can be applied to spread subcarriers over thefrequency domain, in contrast with spreading over the time domain aswith conventional CDMA systems. Applying a Walsh transform before anyIFFT circuit can reduce average power for enhanced LPI/LPD. Variousaspects of the communications system can be readily varied for improvedperformance. With fewer subcarriers as compared to the IFFT size and thespreading sequence length, more processing gain may be realized fromfrequency domain spreading. Furthermore, LPI/LPD and Anti-Jamming (AJ)performance can be enhanced, and there can be higher SNR per subcarrier.

Increasing the sample rate also increases the bandwidth, data rate, andimproves the LPI/LPD/AJ performance.

FIG. 5 is a graph representing a three-dimensional spectrogram of thesymbol-based, frequency randomized subcarriers and showing a comparisonin log scale with a magnitude vs. frequency vs. time representation 501.A conventional single frequency carrier signal 502 is overlaid forcomparison and illustrated as a single carrier toward the lowerfrequency end of the band. This single carrier signal acts similarly toa jammer or interferer. The reduced Inter-Carrier Interference (ICI) isshown by an increased frequency carrier spacing. Reduced Inter-SymbolInterference (ISI) is shown by increased symbol spacing per frequency.This ensures that consecutive OFDM symbols subcarriers do not use thesame frequency and the adverse effects from multipath delay spread areavoided. The same Instantaneous Signal-to-Noise ratio (SNR) as a singlecarrier is also illustrated.

Referring now to FIGS. 6 and 7, there are illustrated respectivefunctional block diagrams for a transmitter 100 (FIG. 6) and a receiver200 (FIG. 7) that can be used in accordance with non-limiting examplesof the present invention. The transmitter 100 as illustrated applies afrequency hopping algorithm to OFDM subcarriers and a frequency domainspreader, for example a Walsh transform, before an IFFT circuit.

Many of the high-level components of the illustrated transmitter 100 andreceiver 200 are functionally similar to the components shown in theprior art modem of FIGS. 1A and 1B, but with further details andfunctional block components added to the transmitter and receiver blockdiagrams shown in FIGS. 6 and 7. For reference purposes, the descriptionfor the transmitter begins with the reference numerals in the 100 seriesand the description for the receiver begins with reference numerals inthe 200 series.

Added functional components that aid in generating the frequencyhopping, OFDM signal that can be Walsh transformed in accordance with anon-limiting example of the present invention include a Pseudo-RandomAmplitude and Phase Generator 102 and Pseudo-Random Subcarrier Locationscircuit 104. Both the Generator 102 and circuit 104 are operative withan Encryption Algorithm 106 and a Cryptographic and Key generatorcircuit (Crypto-Key) 108 and Master Clock 110. These components can begenerally referred to as an encrypted pseudo-random signal generator. AFrequency Domain Spreader circuit 112 is located before an IFFT circuit,as illustrated, and is operable for frequency spreading the signal, suchas by applying a Walsh transform. Also, a digital/analog converter canreceive a signal from a Bandwidth Adjust DAC Sample Rate circuit 114 forremoving spectral lines. These components are explained in furtherdetail below.

As illustrated in FIG. 6, a signal is received within a data buffer 120and passes through a CRC generator 121 and data scrambler 122. An FECencoder circuit shown by the dashed lines at 124 can include a ForwardError Correction encoder 126, for example, a convolutional encoder andpuncturer circuit 128. The encoded signal is interleaved within aninterleaver circuit 130. The signal passes into a modulation and symbolmapping circuit shown generally by the dashed lines at 132. Thismodulation and symbol mapping circuit 132 includes a QAM/PSK modulator134 and Insert Pilot Carriers and PAPR Reduction Carriers circuit 136that inserts pilot carriers and PAPR reduction carriers into the signal.PAPR in this example corresponds to Peak-to-Average Power Ratio.Carriers are mapped to the IFFT in a matrix operation in a subcarriermapper circuit 138.

The Encryption Algorithm 106 is operative not only with the Crypto-Keycircuit 108 and the Master Clock 110, but also the Pseudo-RandomAmplitude and Phase Generator 102, which generates pseudo-random signalsto the QAM/PSK Modulator 134 in accordance with a non-limiting exampleof the present invention. The Pseudo-Random Subcarrier Location circuit104 is also operative with the Subcarrier Mapper circuit 138 andreceives signals from the Encryption Algorithm 106. The OFDM Subcarriersare frequency hopped quickly by means of such circuits.

In accordance with a non-limiting example of the present invention, theFrequency Domain Spreader circuit 112 is located before the IFFT circuit140 and applies the Walsh transform in the frequency domain. If theFrequency Domain Spreader circuit 112 were located after the IFFTcircuit 140, then the Walsh or other function would force a time-domainspreading. It should be understood that the Frequency Domain Spreadercircuit 112 and IFFT circuit 140 can typically be considered with themodulation and mapping circuit 132 as an OFDM modulation circuit or OFDMmodulation and mapping circuit. In accordance with a non-limitingexample of the present invention, the spreading resulting fromapplication of the Walsh transform occurs in the frequency domain. Acyclic extension as a guard interval can be added within a CyclicExtension circuit 142. A symbol-shaping filter 144 such as a FiniteImpulse Response (FIR) filter, cosine filter, or raised cosine filtercan be operative as a “Time Window” for symbol shaping in conjunctionwith the Cyclic Extensions. A packet buffer 146 receives the signals,and after buffering, the signals are converted to analog signals in adigital/analog converter 148. The D/A converter 148 also receives from aBandwidth Adjust DAC Sample Rate circuit 114 a signal for furtherprocessing that removes spectral lines. The D/A Converter 148 passessignals to a Radio Integrated Circuit (IC) Wideband Slow FrequencyHopping circuit 150. The RF carrier can be subjected to a pseudo-randomfrequency hopping algorithm for enhanced bandwidth, and is operativealso as a frequency up-converter, as illustrated.

Basic components of the frequency up-converter circuit 150 can include atransmit chain circuit 152 that receives the signal into a mixer 154.The signal passes through a Bandpass Filter 156, a series of amplifiers158 and through a Single Pole Double Throw (SPDT) switch 160. Afterswitching, a low pass filter 162 filters the signal. The radio frequencysignal is amplified by the Power Amplifier 164 for subsequenttransmission through antenna 166. Other components in the circuit 150include a Phased-Lock Loop circuit 170, a 40. MHz signal generator 172as a non-limiting example, a low pass filter 174, an amplifier 176, asynthesizer 170, another amplifier 180, a bandpass filter 182, a summercircuit 184, and another amplifier 186 that connects to the mixer 154.The component parts of frequency upconverter circuit 150 may be used toaffect a low rate frequency hopping scheme, where the entire OFDMbaseband waveform is frequency translated to different centerfrequencies. Such slow frequency hopping can further guard againstinterference and provide an additional level of encryption if the slowhopping sequence is designed as such.

The transmitter 100 as described is a non-limiting example and manyother types of transmitters could be used. It should be understood thatwith advances in DSP and other circuit functions, processing canpossibly occur directly at baseband.

It should also be understood that the subcarrier mapper circuit 138 mapscarriers to the IFFT circuit 140. For example, if the IFFT circuit 140has an input with a 64 sample signal in the frequency domain, it wouldgive a 64 sample signal in the time domain as a matrix operation. Thesubcarrier mapper circuit 138 can change the order of the vectors toposition symbols on arbitrary subcarriers and apply zero to othersubcarriers. For example, some of the samples in a 64 sample vectorwould be zeros, meaning they would not show up in the frequency domainif they are OFF. Any that are ON or non-zero will change location withevery IFFT cycle (once per symbol) to produce the frequency hopping OFDMsignal. The nature of the frequency hopping for the OFDM signal isgenerated by the Encryption Algorithm 106 and the Pseudo-RandomSubcarrier Locations circuit 104 and the Pseudo-Random Amplitude andPhase Generator 102. The QAM/PSK Modulator 134 aids in producing theconstellation amplitude and phase.

One of the aspects of this invention involves obscuring to an unintendedreceiver that the data has been encrypted. To obscure the encryption,three unknowns are produced by the transmitter. For example, there is a)the unknown of the transmitted amplitude and phase; b) the unknown ofthe pseudo-random amplitude and phase; and c) the unknown of the channelamplitude and phase. Because there are three unknowns, it is notpossible to know which signal is transmitted with an encryptionalgorithm, based on the Cryptographic Key and Master Clock.

The Frequency Domain Spreader circuit 112 operates as a matrixoperation. For example, if a 64 IFFT circuit 140 is employed, then a64×64. Walsh Matrix (as a non-limiting example) can be used tofrequency-spread the subcarriers and provide processing gain. An inputvector would be multiplied by the Walsh matrix. It should be understoodthat a Walsh matrix is a square matrix with dimensions that can be apower of “two.” The entries are positive or negative one (+1, −1). TheWalsh matrix can be obtained from a Hadamard Matrix that is defined by arecursive formula of the same dimension by arranging rows such that thenumber of sign changes is in increasing order, i.e., sequentialordering. Each row of a Walsh matrix corresponds to a Walsh function.The ordering of rows in a Walsh matrix can be derived from ordering aHadamard matrix by applying a bit-reversal permutation and a Gray codepermutation. The Walsh functions form an orthogonal basis of a squarethat is integratable on a unit interval. Thus, it can generatestatistically unique sets of numbers suitable for use in encryption,also known as “pseudo-random and noise codes.” The multiplication may beimplemented efficiently as a series of additions and subtractions.

The Bandwidth Adjust DAC Sample Rate circuit 114 is operative with theD/A converter 148 and can adjust the sample rate and remove spectrallines. As a result, it is harder to detect the waveform with aSpectrogram. It should be understood that the transmitter 100 asdescribed is operative to form a Frequency Hopping OFDM signal with aWalsh transform. For example, if an IFFT is used with 64 samples persymbol, the frequency location of each subcarrier can be changed every64 samples. As an example, if an IFFT is computed every fourmicroseconds, then frequency hopping on all 64 carriers can occur everyfour microseconds to impart a fast hopping rate. Because this can beaccomplished symbol-by-symbol, the frequency hopping OFDM communicationssystem as described can also be termed a Symbol-Based Randomized OFDMbecause the subcarrier frequency locations are randomly changed. Anotherreceiver would not be able to determine the subcarrier locations withoutthe Encryption Algorithm and related circuits, and a fullsynchronization.

FIG. 7 shows a high-level functional block diagram of a receiver 200that can be used in accordance with a non-limiting example of thepresent invention. Similar components that are used in the block diagramof FIG. 6, such as an Encryption Algorithm circuit, Cryptographic Keycircuit, Master Clock, Pseudo-Random Amplitude and Phase Generator,Pseudo-Random Subcarrier Locations circuit, and Bandwidth Adjust ADCSample Rate circuit are given similar reference numerals as used in FIG.6, except they are now placed in the 200 series. This receiver circuit200 also includes the addition of a Symbol-Based SubcarrierSynchronization circuit 216. It also uses a Frequency Domain Despreadercircuit 212 instead of a Frequency Domain Spreader circuit 112 as in thetransmitter 100 of FIG. 6.

Other high-level components illustrated for this receiver circuit 200include an antenna 220, a low noise amplifier (LNA) 222, and RadioIntegrated circuit down-converter 224, which can process a frequencyhopping carrier signal in reverse if it had been processed for widebandby the Radio IC Wideband Slow Frequency Hopping circuit 150 shown in thetransmitter 100 of FIG. 6. The Analog/Digital Converter 226 receives anIF or baseband signal from the down-converter 224, and a signal from theBandwidth Adjust ADC Sample Rate circuit 214 and reverses the processused at the transmitter 100. The signal is forwarded to the Data Buffer228 and Symbol-Based Subcarrier Synchronization circuit 216, whichsynchronizes the subcarriers for further processing. The Guard Intervalcircuit 230 removes the guard interval and the signal is processed withthe Fast Fourier Transform as an OFDM demodulator in an FFT circuit 232.The Inverse Walsh Transform is applied in an Inverse Walsh Transformcircuit 212, A subcarrier demapper and demodulation circuit is shown bydashed lines at 234 and performs an inverse mapping operation to thesubcarriers in subcarrier demapper circuit 236, removes the pilot tonesin a pilot remove circuit 238 and demodulates the signal in a Symbol toNumber (QAM/PSK) Demodulator circuit 240. The deinterleaver circuit 242deinterleaves the signal. A decoding circuit is shown by dashed lines at244 and is operative for depuncturing within depuncture circuit 246 andForward Error Correction (FEC) decoding such as Viterbi decoding withina FEC decoder such as a Viterbi decoder 248. Data descrambling occurs ata Data Descrambler 250, followed by data buffering in data buffer 252and processing for a CRC check by CRC circuit 254.

The transmitter 100 and receiver 200 shown in FIGS. 6 and 7 can generateand receive a signal that is a fast-carrier frequency hopping signal.This hopping can be much faster than a conventional Bluetooth systemthat hops frequencies at 1600 hops/second over a 80 MHz radio frequencybandwidth using a single carrier having a 1 MHz bandwidth. It shouldalso be understood, for example, as shown in the graphs of FIG. 4, thata change in signal-to-noise ratio (S/N) can be based on the number ofsubcarriers and can be used as a method of varying the range of theinstantaneous subcarrier signal-to-noise ratio versus that data rate inan adaptive wireless communications system.

For example, the receiver 200 could measure the received Signal-to-Noiseratio per subcarrier, for example, by using channel estimation symbols,a preamble, or a special channel estimation packet. Information can bepassed back to the transmitter as a “channel mask”, specifying thenumber of subcarriers to “turn-off” and the possible frequency locationsof interferers as channel impairments such that the transmitter 100could use the negotiated channel mask to avoid transmission on anyundesirable frequencies. In one example, ten carriers are turned onsimultaneously over a 100 MHz bandwidth, and each carrier is transmittedfor 640 nanoseconds (corresponding to a 1/FFT rate), such that eachcarrier can hop 1,562,500 times per second. This is about a one thousandtimes faster hopping than the Bluetooth protocol and can provide morethan ten times the data rate.

The transmitter 100 can create multiple subcarriers located at specificfrequencies and can generate a pseudo-random frequency hop for eachsubcarrier frequency by applying the frequency hopping algorithm asexplained before. The IFFT circuit 140 creates multiple subcarrierslocated at specific frequencies. In accordance with a non-limitingexample of the present invention, only a small subset of all possiblesubcarriers need to be used at any one time, although all subcarrierscan be used if necessary. For example, as in the example discussedabove, instead of 64 subcarriers, only 10 subcarriers can be used inthis non-limiting example, giving in that example the 1,562,500 hops persecond.

The subcarrier center frequencies can be changed at the OFDM symbol rateusing the encryption algorithm for the pseudo-random frequencies. Thisoccurs at the modulation and mapping circuit 132 in which the carriersare mapped to the IFFT. The center frequencies of the subcarriers canappear random because of the frequency hopping algorithm. The symboltime duration can be very short as noted above, and therefore, eachsubcarrier would appear for a short time at any specific frequency.

The guard time can be reduced or eliminated by ensuring that consecutivesymbols do not contain subcarriers at the same frequency location. Forexample, in prior art systems, if two symbols are back-to-back on thesame frequency, multipath signals could arrive at different times at thesame location. By using the system and circuits shown in FIGS. 6 and 7,these signals do not appear on the same frequency and the signal wouldtypically not be affected by multipath, thus preventing Inter-SymbolInterference (ISI) and substantially reducing required guard time,reducing transmission overhead, and increasing data rate.

It is possible using the transmitter 100 and receiver 200 as shown inFIGS. 6 and 7 to eliminate or substantially reduce guard time, e.g.“guard interval.” Also, it should be understood that an additional guardcan be added by modifying the frequency hopping algorithm such that nofrequency can be used twice in a row for consecutive symbols, andthereby preventing Inter-Symbol Interference (ISI) because of multipathchannel effects. As noted before, this eliminates or substantiallyreduces the required guard interval, reduces transmission overhead, andincreases the data rate.

It is also possible to dynamically add and remove subcarriers dependingon the required data rate. The minimum carrier spacing can increase toreduce the Inter-Carrier Interference (ICI) and provide robustness tojamming i.e. anti-jamming (AJ) capability, because of the frequencyhopping signal. As long as carriers are not transmitted next to eachother in the frequency domain, the Inter-Carrier Interference will bereduced.

It is also possible for the carrier frequency to hop pseudo-randomly andcover a wide bandwidth. This can be accomplished by the Radio ICWideband Slow Frequency Hopping circuit 150 shown in FIG. 6 andoperative as a frequency up-converter circuit.

A “dead-time” pseudo-random generator can be introduced into the systemto decrease “on” time, and the output spacing between symbols can beincreased. The spacing can be varied using the pseudo-random generatorto prevent spectral lines and reduce cyclostationary statistics of thesignal. This type of system can be implemented without an output samplecontrol. The system can wait a random amount of time beforetransmitting. By removing the spectral lines, it is more difficult forother systems to detect the transmitted communications. The termcyclostationary can refer to the mean standard deviation as a secondorder statistic of the signal. The output sample control can refer tocontrol at the D/A Converter 148.

The subcarrier constellation amplitude and phase values can also bevaried pseudo-randomly using the generator 102 operative with themodulator 134 as shown in FIG. 6. For example, pseudo-random amplitudeand phase values can be generated using the encryption algorithm. Thepseudo-random amplitude and phase values can be added to the intendedamplitude and phase values before transmission. By adding thepseudo-random amplitude and phase values to each subcarrier, the symbolconstellation is no longer a standard QAM/PSK. If the transmitter signalis detected by an unintended receiver, that receiver would not be ableto demodulate the signal because there would be too many unknowns. Forexample, the transmitted or intended amplitude and phase would be anunknown, together with the pseudo-random amplitude and phase that isadded to the signal, and a further unknown being the channel amplitudeand phase for the multipath. This results in three unknowns. Thepseudo-random amplitude and phase values would appear as a typicalrandom channel effect to the unauthorized or unintended receiver.

It should be understood that these algorithms can be added to SoftwareDefined Radios (SDR) and can be implemented with some changes to varydata rate and modulation. The data rates, bandwidth, transmission powerand LPI/LPD performance can be improved by varying the subcarriermodulation scheme, sample rate, IFFT size, IFFT duration and the numberof subcarriers used per OFDM symbol.

As shown in FIG. 6, a Walsh transform can be applied in the frequencydomain for frequency spreading, since it is applied before the IFFTcircuit 140 using the Frequency Domain Spreader circuit 112. It is knownthat Walsh transforms are typically used in communications systems suchas CDMA for time-domain spreading and for creating orthogonal codes formultiple access schemes. The Walsh Transform can be used in the system,apparatus, and method of the present invention to spread subcarriersover the frequency domain. This can provide a significant reduction inthe average power (dBm/Hz/sec) for enhanced LPI/LPD performance,allowing more transmit power within the same FCC spectral mask andreducing the effect of Frequency Selective Fading by providing afrequency-domain processing gain. It also provides additionalanti-jamming (AJ) robustness. Also, out-of-band noise (OBN) emissionscan be reduced similar to time-windowing because of the steeper“roll-off” caused by the Walsh transform. The Walsh transform as amatrix is made up of only positive and negative ones (+1, −1) andrequires only additions and subtractions, and no multiplications. Thiswould allow a trade-off for the number of carriers versus the data rateversus the transmit power and distance for the same FCC spectral mask.In the Walsh transform, matrix rows can be exchanged with each other.The transform would still be orthogonal at the receiver 200. These rowpermutations can be performed to increase further the LPI.

It should be understood that OFDM is susceptible to Frequency SelectiveFading because of multipath. The Walsh transform can provide processinggain to the system and robustness against frequency selective fading.

The system, apparatus and method as described provides a very fastfrequency hopping by changing subcarrier frequency locations, forexample, at the OFDM symbol rate. Thus, it can provide a reducedspectral density over time (decibel/hertz/second) in order to provide aLow Probability of Interception (LPI) and Low Probability of Detection(LPD). The system as described is much faster than Bluetooth systems,and makes the transmission within the FCC spectral mask possible atgreater distances. It also eliminates or substantially reduces a guardinterval by ensuring that subcarriers do not appear on the samefrequency for consecutive OFDM symbols. The system also providesrobustness against Inter-Symbol Interference (ISI) due to multipath. TheWalsh transform can be applied in the frequency-domain to spread thefrequency-hopping subcarriers over the spectrum and reduce the powerspectral density (decibels over hertz) to improve LPI/LDP performance orhelp comply with FCC spectral mask requirements. It can also provide aprocessing gain against frequency selective fading and providerobustness against jamming.

Referring now to FIGS. 8A and 8B, there are shown a spectral comparisonbetween a conventional single-carrier waveform and the symbol-baserandomized frequency hopping subcarriers. As shown in the upper graph,the frequency is on the horizontal axis and the relative power indecibels is on the vertical axis. A spectrum is shown at baseband andthe average intensity is illustrated together with the 30 decibel LPDimprovement over a conventional system. More power can now betransmitted within the same FCC spectral mask. It is possible for aradio station or other transmitter that transmits digital data tofrequency hop its OFDM signal and reduce the average power in accordancewith a non-limiting example of the present invention.

FIG. 9 graphs show a spectral comparison in noise such as in closeproximity to a transmitter. A single carrier is compared to thefrequency hopping OFDM subcarriers.

FIG. 10 is a graph showing a transmitted frequency hopping OFDM signalin three-dimensions before the Walsh transform in accordance with anon-limiting example of the present invention. The frequency hoppingOFDM signal is illustrated.

FIG. 11 is a three-dimensional graph showing the transmitted frequencyhopping OFDM signal after the Walsh transform in which the power isreduced even more in accordance with a non-limiting example of thepresent invention. Each subcarrier has the Walsh transform applied inthe frequency domain. The subcarriers are “smeared” or spread overfrequency to reduce the power per Hertz to a greater extent than beforethe Walsh transform.

FIG. 12 is a graph showing the addition of the Walsh transform to theSymbol-Based Randomized subcarriers in accordance with a non-limitingexample of the present invention. The Walsh transform is shown when itis OFF and ON, also showing the various differences in power.

FIG. 13 is a graph showing the received frequency hopping OFDM signal inthree-dimensions before the Inverse Walsh transform, and suggesting howdifficult the signal could be to decode without knowing the encryptionalgorithm.

FIG. 14A is a three-dimensional graph showing the received signal fromFIG. 13 after the Inverse Walsh transform is applied, in which thesignal “pops” out and can be decoded. The received signal constellationafter the inverse Walsh transform is shown in the lower right at FIG.14B.

FIGS. 15A and 15S show the frequency hopping OFDM signal before andafter the Walsh transform in which a single carrier system is shown inthe middle of the band at 1501. As shown in the graph of FIG. 11B, afterthe Walsh transform, the single carrier does not have the Walshtransform applied, but the other OFDM signal, subject to the frequencyhopping in accordance with a non-limiting example of the presentinvention, is spread over frequency.

FIG. 16 is a three-dimensional graph showing a frequency-domaindespreading with an interferer in which the interferer is spread acrossthe frequency after the Inverse Walsh transform. Thus, there is afrequency-domain despreading with the interferer.

FIG. 17 shows the transmitted OFDM signal before the Walsh transform ina similar simulation before adding noise. This figure shows thetransmitted signal before application of the frequency-domain spreadingand transmission over the noisy channel.

FIG. 18 shows a received OFDM signal with the interferer before theInverse Walsh transform for a real noise environment and showing theextended interferer signal. The frequency hopping and spread OFDM signalis shown above the noise floor.

FIG. 19 is a graph showing a spectral comparison with the Walshtransform ON and OFF with the interferer and showing the interferer andthe frequency hopping OFDM signal and the location of the Walshtransformed signal.

FIG. 20 is a power spectrum of a received frequency hopping and spreadOFDM signal with the interferer before the Inverse Walsh transform.

FIG. 21 is a graph in two-dimension showing the frequency-domaindespreading with the interferer in which the interferer is spread acrossthe frequency after the Inverse Walsh transform.

FIG. 22 is a graph showing the frequency-domain despreading with theinterferer spread across frequency after the Inverse Walsh transform andshowing a received signal constellation.

An example of a communications system that can be modified for use withthe present invention is now set forth with regard to FIG. 23.

An example of a radio that can be used with such system and method is aFalcon™ III radio manufactured and sold by Harris Corporation ofMelbourne, Fla. The Falcon™ III can include a basic transmit switch, andother functional switches and controls known to those skilled in theart. It should be understood that different radios can be used,including but not limited to software defined radios that can betypically implemented with relatively standard processor and hardwarecomponents. One particular class of software radio is the Joint TacticalRadio (JTR), which includes relatively standard radio and processinghardware along with any appropriate waveform software modules toimplement desired communication waveforms. JTR radios also use operatingsystem software that conforms to the software communicationsarchitecture (SCA) specification (see www.jtrs.saalt.mil), which ishereby incorporated by reference in its entirety. The SCA is an openarchitecture framework that specifies how hardware and softwarecomponents are to interoperate so that different manufacturers anddevelopers can readily integrate the respective components into a singledevice.

The Joint Tactical Radio System (JTRS) Software Component Architecture(SCA) defines a set of interfaces and protocols, often based on theCommon Object Request Broker Architecture (CORBA), for implementing aSoftware Defined Radio (SDR). In part, JTRS and its SCA are used with afamily of software re-programmable radios. As such, the SCA is aspecific set of rules, methods, and design criteria for implementingsoftware re-programmable digital radios.

The JTRS SCA specification is published by the JTRS Joint Program Office(JPO). The JTRS SCA has been structured to provide for portability ofapplications software between different JTRS SCA implementations,leverage commercial standards to reduce development cost, reducedevelopment time of new waveforms through the ability to reuse designmodules, and build on evolving commercial frameworks and architectures.

The JTRS SCA is not a system specification, as it is intended to beimplementation independent, but a set of rules that constrain the designof systems to achieve desired JTRS objectives. The software framework ofthe JTRS SCA defines the Operating Environment (OE) and specifies theservices and interfaces that applications use from that environment. TheSCA OE comprises a Core Framework (CF), a CORBA middleware, and anOperating System (OS) based on the Portable Operating System Interface(POSIX) with associated board support packages. The JTRS SCA alsoprovides a building block structure (defined in the API Supplement) fordefining application programming interfaces (APIs) between applicationsoftware components.

The JTRS SCA Core Framework (CF) is an architectural concept definingthe essential, “core” set of open software Interfaces and Profiles thatprovide for the deployment, management, interconnection, andintercommunication of software application components in embedded,distributed-computing communication systems. Interfaces may be definedin the JTRS SCA Specification. However, developers may implement some ofthem, some may be implemented by non-core applications (i.e., waveforms,etc.), and some may be implemented by hardware device providers.

For purposes of description only, a brief description of an example of acommunications system that would benefit from the present invention isdescribed relative to a non-limiting example shown in FIG. 23. This highlevel block diagram of a communications system 350 includes a basestation segment 352 and wireless message terminals that could bemodified for use with the present invention. The base station segment352 includes a VHF radio 360 and HF radio 362 that communicate andtransmit voice or data over a wireless link to a VHF net 364 or HF net366, each which include a number of respective VHF radios 368 and HFradios 370, and personal computer workstations 372 connected to theradios 368, 370. Ad-hoc communication networks 373 are interoperativewith the various components as illustrated. Thus, it should beunderstood that the HF or VHF networks include HF and VHF net segmentsthat are infrastructure-less and operative as the ad-hoc communicationsnetwork. Although UHF radios and net segments are not illustrated, thesecould be included.

The HF radio can include a demodulator circuit 362 a and appropriateconvolutional encoder circuit 362 b, block interleaver 362 c, datarandomizer circuit 362 d, data and framing circuit 362 e, modulationcircuit 362 f, matched filter circuit 362 g, block or symbol equalizercircuit 362 h with an appropriate clamping device, deinterleaver anddecoder circuit 362 i modem 362 j, and power adaptation circuit 362 k asnon-limiting examples. A vocoder (voice encoder/decoder) circuit 362 lcan incorporate the encode and decode functions and a conversion unitwhich can be a combination of the various circuits as described or aseparate circuit. A transmit key switch 362 m is operative as explainedabove. These and other circuits operate to perform any functionsnecessary for the present invention, as well as other functionssuggested by those skilled in the art. The circuits referenced here mayinclude any combination of software and/or hardware elements, includingbut not limited to general purpose microprocessors and associatedsoftware, specialized microprocessors for digital signal processing andtheir associated software, Application Specific Integrated Circuits(ASICs), Field Programmable Gate Arrays (FPGAs), logic circuits, orother kinds of devices and/or software or firmware known to thoseskilled in the art. Other illustrated radios, including all VHF mobileradios and transmitting and receiving stations can have similarfunctional circuits.

The base station segment 352 includes a landline connection to a publicswitched telephone network (PSTN) 380, which connects to a PABX 382. Asatellite interface 384, such as a satellite ground station, connects tothe PABX 382, which connects to processors forming wireless gateways 386a, 386 b. These interconnect to the VHF radio 360 or HF radio 362,respectively. The processors are connected through a local area networkto the PABX 382 and e-mail clients 390. The radios include appropriatesignal generators and modulators. The packetized or non-packetizeddigital voice information transmitted within the network using thetechniques of the present invention can originate at or be delivered toa handset connected to one of the radios, a telephone or other interfacedevice attached to a wireless gateway device such as the RF-6010.Tactical Network Hub, or a subscriber telephone connected to the PABX orwithin the public switched telephone network.

An Ethernet/TCP-IP local area network can operate as a “radio” mailserver. E-mail messages can be sent over radio links and local airnetworks using STANAG-5066 as second-generation protocols/waveforms, thedisclosure which is hereby incorporated by reference in its entiretyand, of course, preferably with the third-generation interoperabilitystandard: STANAG-4538, the disclosure which is hereby incorporated byreference in its entirety. An interoperability standard FED-STD-1052,the disclosure which is hereby incorporated by reference in itsentirety, can be used with legacy wireless devices. Examples ofequipment that can be used in the present invention include differentwireless gateway and radios manufactured by Harris Corporation ofMelbourne, Fla. This equipment includes RF5800, 5022, 7210, 5710, 6010,5285 and PRC 117 and 138 series equipment and devices as non-limitingexamples.

These systems can be operable with RF-5710A high-frequency (HF) modemsand with the NATO standard known as STANAG 4539, the disclosure which ishereby incorporated by reference in its entirety, which provides fortransmission of long distance HF radio circuits at rates up to 9,600bps. In addition to modem technology, those systems can use wirelessemail products that use a suite of data-link protocols designed andperfected for stressed tactical channels, such as the STANAG 4538rSTANAG 5066, the disclosures which are hereby incorporated by referencein their entirety. It is also possible to use a fixed, non-adaptive datarate as high as 19,200 bps or higher with a radio set to ISB mode and anHF modem set to a fixed data rate. It is possible to use code combiningtechniques and ARQ.

This application is related to copending patent applications entitled,“SYSTEM AND METHOD FOR COMMUNICATING DATA USING SYMBOL-BASED RANDOMIZEDORTHOGONAL FREQUENCY DIVISION MULTIPLEXING (OFDM),” and “SYSTEM ANDMETHOD FOR APPLYING FREQUENCY DOMAIN SPREADING TO MULTI-CARRIERCOMMUNICATIONS SIGNALS,” and “SYSTEM AND METHOD FOR COMMUNICATING DATAUSING SYMBOL-BASED RANDOMIZED ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING(OFDM) WITH SELECTED SUBCARRIERS TURNED ON OR OFF,” and “METHOD OFCOMMUNICATING AND ASSOCIATED TRANSMITTER USING CODED ORTHOGONALFREQUENCY DIVISION MULTIPLEXING (COFDM),” which are filed on the samedate and by the same assignee and inventors, the disclosures which arehereby incorporated by reference.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

1. A device for communicating data, comprising: a modulation and mapping circuit that modulates and maps data symbols into a plurality of multiple subcarrier frequencies that are orthogonal to each other to form an Orthogonal Frequency Division Multiplexed (OFDM) communications signal based on a fixed or variable OFDM symbol rate; a pseudo-random signal generator operative with the modulation and mapping circuit for generating pseudo-random signals to the modulation and mapping circuit based on an encryption algorithm for frequency hopping each subcarrier at an OFDM symbol rate to lower any probability of interception and detection, reduce power per frequency (dB/Hz/sec), and lower any required transmission power while maintaining an instantaneous signal-to-noise ratio; and a frequency domain spreader circuit operatively connected to said modulation and mapping circuit for spreading the multiple subcarriers over the frequency domain.
 2. The device according to claim 1, wherein said frequency domain spreader circuit comprises a Walsh Transform circuit that is operative for applying a Walsh Transform and spreading said multiple subcarriers over the frequency domain.
 3. The device according to claim 2, wherein said Walsh Transform circuit is operative for multiplying an input vector of a symbol by the Walsh Transform.
 4. The device according to claim 1, and further comprising an Inverse Fast Fourier Transform (IFFT) circuit positioned to receive signals from said frequency domain spreader circuit.
 5. The device according to claim 1, wherein said pseudo-random signal generator is operative for generating a pseudo-random signal based on said encryption algorithm such that consecutive OFDM symbols do not transmit subcarriers on the same frequency.
 6. The device according to claim 1, wherein said pseudo-random signal generator is operative for generating a pseudo-random signal based on said encryption algorithm such that OFDM symbols do not transmit subcarriers on adjacent frequencies for reduced Inter-Carrier Interference (ICI).
 7. The device according to claim 1, wherein said pseudo-random signal generator is operative for generating a pseudo-random signal based on said encryption algorithm such that a Guard Interval is reduced or eliminated.
 8. The device according to claim 1, wherein said modulation and mapping circuit is operative for inserting signals for reducing peak-to-average-power ratio (PAPR).
 9. The device according to claim 1, and further comprising a modulator for mapping communications data into modulated symbols based on a specific mapping algorithm.
 10. The device according to claim 1, wherein said pseudo-random signal generator is operatively connected to said modulator for varying amplitude and phase values using an encryption algorithm.
 11. The device according to claim 1, and further comprising a Forward Error Correction circuit for adding a FEC code.
 12. A system for communicating data, comprising: a transmitter for transmitting a communication signal that carries communications data and comprises a modulation and mapping circuit that maps data symbols into a plurality of multiple subcarrier frequencies that are orthogonal to each other to form an Orthogonal Frequency Division Multiplexed (OFDM) communications signal based on a fixed or variable OFDM symbol rate, and a pseudo-random signal generator operative with the modulation and mapping circuit for generating pseudo-random signals to the modulation and mapping circuit based on an encryption algorithm for frequency hopping each subcarrier at an OFDM symbol rate to lower any probability of interception and detection, reduce power per frequency (dB/Hz/sec), and lower any required transmission power while maintaining an instantaneous signal-to-noise ratio, and a frequency domain spreader circuit operatively connected to said modulation and mapping circuit for spreading the multiple subcarriers over the frequency domain; and a receiver for receiving the communications signal and including a demapping and demodulation circuit and frequency domain despreading circuit for processing the communications signal to obtain the communications data.
 13. A system according to claim 12, wherein said transmitter is operative for modulating a main carrier signal.
 14. The system according to claim 12, wherein said frequency domain spreader circuit comprises a Walsh Transform circuit that is operative for applying a Walsh Transform for spreading said multiple subcarriers over the frequency domain.
 15. The system according to claim 14, wherein said Walsh Transform circuit is operative for multiplying an input vector of a symbol by the Walsh Transform.
 16. The system according to claim 12, and further comprising an Inverse Fast Fourier Transform (IFFT) circuit positioned to receive signals from said frequency domain spreader circuit.
 17. The system according to claim 12, wherein said pseudo-random signal generator is operative for generating a pseudo-random signal based on said encryption algorithm such that consecutive OFDM symbols do not transmit subcarriers on the same frequency.
 18. The system according to claim 12, wherein said pseudo-random signal generator is operative for generating a pseudo-random signal based on said encryption algorithm such that OFDM symbols do not transmit subcarriers on adjacent frequencies for reduced Inter-Carrier Interference (ICI).
 19. The system according to claim 12, wherein said pseudo-random signal generator is operative for generating a pseudo-random signal based on said encryption algorithm such that a Guard Interval is reduced or eliminated.
 20. The system according to claim 12, wherein said pseudo-random signal generator is operatively connected to said modulator for varying amplitude and phase values using an encryption algorithm.
 21. The system according to claim 12, wherein said transmitter further comprises a circuit for inserting signals for reducing peak-to-average-power ratio (PAPR).
 22. The system according to claim 12, wherein said transmitter is operative for adding a Forward Error Correction (FEC) code.
 23. A method for communicating data, which comprises: distributing communications data within a modulation and mapping circuit over multiple subcarriers that are orthogonal to each other to form an Orthogonal Frequency Division Multiplexed (OFDM) communications signal based on fixed or variable OFDM symbol rate; frequency hopping each subcarrier within a pseudo-random signal generator at an OFDM symbol rate to lower any probability of interception and detection, reduce power per frequency (dB/Hz/sec), and lower any required transmission power while maintaining an instantaneous signal-to-noise ratio; spreading the multiple subcarriers over the frequency domain within a frequency domain spreader circuit; and transmitting the communications data over a communications signal that includes the frequency hopping subcarriers.
 24. The method according to claim 23, which further comprises applying a Walsh Transform for spreading the multiple subcarriers over the frequency domain.
 25. The method according to claim 24, which further comprises multiplying an input vector of a symbol by a Walsh Transform.
 26. The method according to claim 23, which further comprises applying a frequency spreading function before an Inverse Fast Fourier Transform (IFFT).
 27. The method according to claim 23, which further comprises generating a pseudo-random signal based on an encryption algorithm such that consecutive OFDM symbols do not transmit subcarriers on the same frequency.
 28. The method according to claim 23, which further comprises generating a pseudo-random signal based on an encryption algorithm such that OFDM symbols do not transmit subcarriers on adjacent frequencies.
 29. The method according to claim 23, which further comprises generating a pseudo-random signal based on said encryption algorithm such that a Guard Interval is reduced or eliminated.
 30. The method according to claim 23, which further comprises inserting signals for reducing peak-to-average-power ratio (PAPR).
 31. The method according to claim 23, which further comprises modulating a main carrier signal on which the multiple subcarrier frequencies are transmitted.
 32. The method according to claim 23, which further comprises varying pseudo-randomly a subcarrier constellation amplitude and phase value.
 33. The method according to claim 23, which further comprises adding a Forward Error Correction code. 